1. Field of the Invention
The invention relates generally to the field of photolithography processes, and specifically to contrast enhancement techniques.
2. Prior Art
As the scale of integrated circuits decrease, it becomes increasingly more difficult to manufacture these devices. The most common means used to transfer patterns for devices from a master mask to a semiconductor wafer is a mask aligner with a light source which operates in the near ultra-violet and blue regions of the spectrum. The light shines through the mask, selectively exposing areas of a photosensitive polymer, or photoresist. The exposed areas have different solubility characteristics relative to the unexposed areas, and the pattern can be developed out in an appropriate solvent. Unfortunately, optical mask aligners have difficulty resolving features with dimensions that are less than one micron.
The resolution limits of the optical exposure means can be extended by using a technique called contrast enhancement lithography. The contrast enhancement technique uses a two layer photolithographic process. The bottom layer is a conventional positive AZ photoresist which has as its polymer base a phenol formaldehyde (novolak) resin. The top layer, or contrast enhancement layer, is a polymeric coating which contains a dye that bleaches at the wavelength of the optical exposure apparatus. This results in a reduction of the contrast threshold of the photolithographic system, leading to an increase in the maximum resolvable spatial frequency. Thus the resolution limits of the optical exposure system are extended. All such prior art contrast enhancement layers may only be dissolved by an organic or hydrocarbon solvent which may be harmful to the user, difficult to dispose of, and represents a separate processing step.